Agriculture Reference
In-Depth Information
4.7 OM VERSUS SOIL BIOLOGICAL PROPERTIES
SOM contents significantly influence the soil biological properties such as nitrogen mineralization
bacteria, dinitrogen-fixing bacteria, mycorrhizae fungi, and total microbial biomass. These proper-
ties are summarized in Table 4.2 and a detailed discussion is given in the succeeding sections.
4.7.1 n
ItroGen
-m
IneralIzInG
B
aCterIa
Nitrogen mineralization is the conversion of organic N into inorganic N by microbial activity. Urea
and ammonium sulfate are dominant nitrogen carriers used for crop production around the world.
The oxidation of the ammonium form of nitrogen fertilizers, which form NO
3
−
, can be explained by
the following equation:
NH
+
+ ⇔+
2
O
OHO H
−
+
+
4
2
3
2
The oxidation of NH
4
+
in the above equation is known as nitrification and heterotrophic and
autotrophic bacteria can carry it out. The most important autotrophic genera of bacteria are
Nitrosomonas
and
Nitrobacter
. An adequate quantity of OM in the soil reduces the soil acidity and
improves activities of these nitrogen mineralization bacteria. With the reduction of soil acidity, there
is improvement in the nodule formation of clover by an indigenous rhizobial strain (Almendras and
Bottomley, 1987; Howieson et al., 1993). OM also influences mineralization of N through its higher
water-holding capacity. Since nitrifying bacteria are generally more sensitive to water deficits than
fungi, the bacteria-dependent nitrification process (
NH
4
+
⇒
NH
2
−
⇒
NH
3
−
) may essentially cease to
operate in a dry soil, whereas the ammonification step (inorganic N ⇒
NH
4
+
), accomplished pre-
dominantly by more drought-tolerant fungi, may still proceed (Power, 1990).
4.7.2 d
enItrIfICatIon
Denitrification is the reduction of nitrogen oxides (usually nitrate and nitrite) to molecular nitrogen
or nitrogen oxides with a lower oxidation state of nitrogen by bacterial activity (denitrification) or by
chemical reactions involving nitrite (chemodenitrification) (Soil Science Society of America, 2008).
Denitrification is one of the major mechanisms for N loss from the soil. Hauck (1981) reported that
denitrification can lose as much as 30% of the applied N under field conditions. The process of
denitrification can be expressed in the form of the following equation:
NO nitrate
−
(
)
⇒
O nitrite
(
)
⇒
NO (nitric oxide)
⇒
(
itric oxi
de
)
3
2
⇒
NO (nitrous oxide)
⇒
N
(
dinitrogen
)
2
2
The majority of denitrification is biologically catalyzed and closely linked to the bacterial respi-
ratory metabolism (Aulakh et al., 1992). In chemodenitrification, generation of N gas is catalyzed by
abiotic agents, but this process may only be of importance in acidic or frozen soils (Christianson and
Cho, 1983). SOM has a significant influence on the denitrification process in the soil-plant system.
In addition, soil pH, temperature, nitrate concentration, aeration, and water status control the deni-
trification rate in the soils. Denitrifying organisms use organic C as electron donors for energy and
for synthesis of cellular constituents (Aulakh et al., 1992). Hence, denitrification strongly depends
on the availability of organic compounds such as native SOM, crop residues, or root biomass. The
higher the organic carbon in the soil, the higher is the denitrification rate (Aulakh et al., 1983, 1984,
1992). The denitrification rate in the soil also depends on the composition of organic residues added
to the soil. The C:N ratio of organic residues is one of the best criteria of its decomposition rate
and supplying energy to the denitrifying bacteria (Aulakh et al., 1992). The lower the C:N ratio, the
